The prevalence of obesity continues to rise, and the striking increases in childhood obesity over the past several decades are particularly alarming. Recent data suggest that 31 percent of children in the U.S. are either overweight or obese and these children at a greater risk for adult obesity and negative health and psychosocial consequences in childhood and adulthood. Although 1/3 of children who participate in family-based obesity treatment decreased their percentage overweight 10 years later, 2/3 of the children continue to struggle with their weight. One potential opportunity for improving childhood obesity treatment is to intervene with the parents, without child involvement. Parent-only interventions are potentially promising for a number of reasons, including decreased treatment costs, developmental appropriateness for the child, promising data, and improved generalizability. This project will fill this gap in the literature, by completing the following primary and secondary aims. Primary aim 1: To evaluate the effectiveness of a behavioral parent-only intervention for childhood obesity on the target child's weight compared to the parent + child group. We hypothesize that the parent-only treatment will produce child weight loss (BMI_Z, BMI-P) that is not inferior to the parent + child treatment group immediately following treatment and at 6-, 12- months and 18- months post-treatment. Primary aim 2: To evaluate the cost effectiveness of a parent-only group, as compared to the parent + child group. We hypothesize that the parent-only group will be more cost-effective than the parent + child group, as measured by calculating cost effectiveness ratios from both third party payer and limited societal perspectives. As secondary aims, we will 1) Compare effect of the treatment groups on child and parent diet and exercise behavior, quality of life, psychosocial measures, parenting skills and 2) Evaluate predictors of success for child weight loss, including group attendance, adherence to behavioral recommendations, household environment, parenting style, parent and child psychosocial functioning, and parent weight loss at 6-, 12- and 18- months post-treatment. Aims will be achieved through a randomized clinical trial in which 150 overweight 8-12 year old children and at least one of their overweight parents will be randomly assigned by the gender of the child to one of two conditions; Parent-only intervention or Parent + child intervention. Both treatment arms will provide behavioral treatment for childhood obesity for 5 months. Assessments include baseline, immediately post-treatment, and 6-, 12- and 18-months post-treatment. Results of these studies will be utilized as data to recommend changes in the standard treatment for childhood obesity. This program of research is critical for exploring and developing interventions mobilizing parents to treat and prevent childhood obesity.